1. Field of the Invention
The present invention relates to a polymeric composite heating element, and more particularly, to a polymeric composite heating element of a quick heating type.
2. Description of the Prior Art
It has been known that by mixing electrically conductive carbon into a heat-resistant polymer an electric resistance heating element can be produced. Since the polymer is used as a matrix material in such electric resistance heating element, the element is excellent in workability, and can be wrought into a heating element in the form of a sheet, pipe, or the like. Thus, developments of various uses thereof have been made. For instance, for a heater in an electric carpet there is used such an electric resistance heating element as a composition comprising polytetrafluoroethylone (hereinafter referred to as "PTFE") and electrically conductive carbon dispersed in it. This electric resistance heating element accomplishes an extremely stable and reliable electrical carpet for its excellent characteristics such as heat-resistance of PTFE and the like, thus preventing any possible troubles.
However, the volume resistivity change of the conventional heating element is extremely small in a temperature range of between room temperature and 200.degree. C. Accordingly, in the case where such heating element is used as a heater in the electric carpet and when power density of such heater is made large, the heater becomes hot quickly after electricity is charged. However, a hunting (oscillation range of the heater temperature at the time of controlling of switch) occurring at the time after a predetermined temperature is reached becomes large, thereby rendering the heater unsuitable for use in the electrical carpet. On the contrary, when the power density of the heater is made small, it is true that the hunting consequently becomes small, but it takes a long time for the carpet to be heated after the switch is turned on.
The quick warming type heating system is the best as the heating system for the electric carpet. The heating system of this type is one which is adapted such that a larger amount of current flows (when the system is initially rendered conductive so as to exhibit a quick heating feature), while during an ordinary operating period a smaller amount of current flows and the power density decreases thereby to reduce the hunting and hence the electric power to be consumed.
It is readily possible for the recent electronic technique to effect the aforementioned heating element of the quick heating type through the control by electric circuits. However, such heating element will be accompanied by the increase in production cost and the controlling space required. Therefore, such element is still unfavorably deficient. Accordingly, the heating element is the most preferable which exhibits a variable resistivity characteristic depending upon a temperature and whose resistance element has a quick heating feature.